Wet oxidation or wet air oxidation is well known and is in commercial use. See for example, U.S. Pat. No. 2,665,249, Zimmermann. Also, the recovery of energy or power in a wet oxidation system is known and has been practiced. See the paper by Morgan and Saul, APPITA, Volume 22, Number 3, "The Zimmermann Process in a Soda Pulp Mill Recovery System". Other references are U.S. Pat. No. 2,944,396, Barton; a paper by Guccione, Chemical Engineering, May 25, 1964, "Wet Combustion of Sewage Sludge Solids Disposal Problems"; and a paper by G. H. Teletzke, Chemical Engineering Progress, Jan. 1964, "Wet Air Oxidation".
The flow sheet of FIG. 1 shows a commercial system that is applicable to relatively concentrated liquors or "strong liquors". These liquors are characterized by a high chemical oxygen demand (COD) with a value of 100 g/l or greater. Examples of such liquors are pulping black liquors and concentrated industrial wastes. COD reduction is generally 90% or more. Distinctive features of the strong liquor flow sheet (FIG. 1) are: The liquid and gas phases are separated immediately after the reactor, e.g., in a separator built into the top of the reactor, and a substantial portion of the water in the feed liquor is evaporated and leaves the reactor in the gas phase. The gases from the reactor are first directed to a steam generator to make pure steam thereby cooling the gases causing a substantial portion of the water vapor to condense, the condensate being then removed as liquid water from a separator. The gas phase is directed to an expander/generator or other device for mechanical power recovery.
A disadvantage of this system is that the expander/generator does not make use of the maximum system temperature. If the gas stream is directed to the expander/generator directly from the reactor then the expander exhaust temperature is too low to enable pure steam to be generated. However, in most commercial applications there is a definite economic advantage in generating pure steam as shown in FIG. 1. A further disadvantage to this system is that when the gas stream from the reactor is cooled to low temperatures in order to make the maximum amount of steam, then expansion of this low temperature gas stream through the expander results in an exhaust temperature well below the freezing point of water which creates serious problems in the expander. It would therefore be necessary to reheat this gas using an outside source of heat.
FIG. 2 is a flow sheet applicable commercially to relatively dilute liquors and sludges or "weaker liquors". The COD of such liquors is less than 100 g/l. An example of such a liquor is sewage sludge which typically has a COD in the range of 40-60 g/l. The degree of COD reduction is typically in the range of 50-85%. There is generally not enough heat in the oxidized liquor alone to accomplish all the necessary preheating in equipment of practical size. Therefore it is necessary to extract heat from the gas phase as well as the liquid phase as in FIG. 2. The entire reactor outlet stream is cooled so that the final temperature of the gas, while still hot, is less than the maximum reactor temperature.
It can be seen that the same preheating effect can be accomplished if the phases are separated immediately after the reactor as shown in FIG. 3. In the flow system of FIG. 3, the gas phase only is passed through the second preheater and thence to a second separator. It can be seen that this system is more complicated requiring two separators and generating two liquid streams, the oxidized liquor and the condensate from the second separator.
In practice, and especially when treating sewage sludge, the gas from the separator from either flow sheet FIG. 2 or FIG. 3 still contains hydrocarbons in sufficient quantities so that further treatment is necessary before the gas can be passed through the expander/generator or discharged to the atmosphere. A catalytic gas-phase oxidizing unit has been used for this purpose.
A typical flow sheet for such a unit is shown in FIG. 4. Gas from the separator goes through a preheater and thence through a fuel fired startup heater, used only to start the device, and then through a catalyst bed where gas phase oxidation of hydrocarbons takes place. The stream exiting from the catalyst is at a higher temperature due to the oxidation and thus can be used to preheat the inlet stream to the catalyst. The gas is then passed to the expander/generator. This system is advantageous under certain circumstances since, for example, it results in discharge of a gas to the expander/generator that is superheated and at a higher temperature than if no gas phase oxidation takes place, and this higher temperature is advantageous to the performance of the expander as is well known. However, with certain feed liquors, especially sewage sludge, many operating problems have arisen. The catalyst is subject to poisoning and fouling and becomes ineffective. The system is complex and expensive since the vessels and piping must be designed for high temperatures. With sewage sludge or other nitrogen containing liquors, nitrogen compounds such as ammonia are present in the gas and these tend to oxidize to nitric acid which creates further problems. Also with certain liquors containing highly volatile substances, volatile hydrocarbons are found in the gases and these have occasionally oxidized in a spontaneous and uncontrolled way in various parts of the system causing failures and creating safety hazards. This problem has been severe enough so that "hot separation" of the gas and liquid phases when treating these liquors probably should be avoided.
A further problem with the FIG. 2 flow sheet is that temperature control is difficult. Temperature control is done by regulating the amount of heat transferred in the preheater according to the system described generally in U.S. Pat. No. 2,903,425, Zimmermann. However, in practice the valves required for this control operation are subject to plugging and erosion and control is difficult and often unsatisfactory.